Defeat FNF's Big Chungus, a really fat version of Bugs Bunny, with whom you can have tons of fun both skinny and fat, especially with an awesome tracklist like this one:
BF VS Big Chungus, an FNF x Looney Tunes crossover!
Make your choice between the story mode and free play mode, and then, in either case, hit your notes according to the charts to reach the end of the songs and win.
So, when you see arrow symbols match above BF's head, that is when you press the identical arrow keys. Don't miss hitting your notes too many times in a row, or you have to start again from scratch. Good luck!